Spark plug

Abstract

A spark plug including a center electrode with a tip welded to a front end portion with a fused portion therebetween. The front end portion contains Ni, Cr, and at least one element selected from a group B consisting of Mn, Si, Al, Ti, rare earth elements, Hf, and Zr. Ni is present in the largest proportion, and Cr is present in the second largest proportion and in an amount of 12% by mass or more. The at least one element selected from the group B is present in a total amount of 0.1% by mass or more. The front end portion satisfies f/e≤0.15 and m/e≤0.015, where f is the Fe content, e is the sum of the Cr, Si, and Al contents, and m is the Mo content.

Having described the invention, the following is claimed: 
     
       1. A spark plug comprising:
 an insulator having formed therein an axial hole extending from front to rear in a direction along an axial line, the insulator including a stop portion overhanging radially outward; 
 a metal shell disposed around the insulator and including a stepped portion protruding radially inward, the stepped portion stopping the stop portion from a front side thereof directly or with another member therebetween; and 
 a center electrode disposed in the axial hole, the center electrode including a front end portion located forward of a front end of the insulator and a tip welded to the front end portion with a fused portion therebetween, 
 wherein the front end portion contains Ni, Cr, and at least one element selected from a group B consisting of Mn, Si, Al, Ti, rare earth elements, Hf, and Zr, Ni being present in the largest proportion, Cr being present in the second largest proportion and in an amount of 12% by mass or more, the at least one element selected from the group B being present in a total amount of 0.1% by mass or more, the front end portion satisfying f/e≤0.15 and m/e≤0.015, where f is an Fe content, e is a sum of Cr, Si, and Al contents, and m is a Mo content, and 
 wherein the spark plug has a distance D of 22 mm or less in the direction along the axial line from a first point located at a frontmost position of a boundary between an outer surface of the front end portion and an outer surface of the fused portion to a second point located at a frontmost position of a contact area between the stepped portion or the other member and the stop portion. 
 
     
     
       2. The spark plug according to  claim 1 , wherein the tip contains Ir in the largest proportion and at least one element selected from a group A consisting of Pt, Ru, Rh, and Ni in an amount of 4% by mass or more. 
     
     
       3. The spark plug according to  claim 1 , wherein
 the front end portion has a region where a plurality of crystal grains appear in a cross-section containing the axial line, 
 a length of the crystal grains in the region in the direction along the axial line is longer than a length of the crystal grains in the region in a direction perpendicular to the axial line, and 
 the front end portion satisfies Ha/Hb≥0.36, where Ha is a Vickers hardness of the region in the cross-section after heat treatment at 900° C. in an Ar atmosphere for 50 hours, and Hb is a Vickers hardness of the region in the cross-section before the heat treatment. 
 
     
     
       4. The spark plug according to  claim 1 , wherein the distance D is 18 mm or less. 
     
     
       5. The spark plug according to  claim 1 , wherein the distance D is 14 mm or less. 
     
     
       6. The spark plug according to  claim 1 , wherein f/e≤0.04. 
     
     
       7. The spark plug according to  claim 1 , wherein m/e≤0.004. 
     
     
       8. The spark plug according to  claim 1 , wherein f/e≤0.001.